Non-Methane Hydrocarbons (NMHCs)

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Researchers from the Aryabhatta Research Institute of Observational Sciences (ARIES) conducted a year-long (2022–2023) study measuring Non-Methane Hydrocarbons (NMHCs) at Munsyari (Uttarakhand) to understand their role in air quality, ozone formation, and climate change.

About Non-Methane Hydrocarbons (NMHCs)

• Non-Methane Hydrocarbons (NMHCs) are a group of volatile organic compounds (VOCs) that exclude methane (CH₄).

• They are present in trace amounts in the atmosphere but play a crucial role in atmospheric chemistry, particularly in the formation of ground-level ozone and secondary organic aerosols (SOAs).

• Common NMHCs include ethane, ethene, propane, propene, butane, pentane, benzene, toluene, xylene, and isoprene.

Sources of NMHCs

Natural Sources

NMHCs are naturally released from forests, vegetation, wetlands, oceans, and soil microorganisms. Plants emit isoprene and terpenes, especially during warm weather, making forests one of the largest natural sources of NMHCs.

Anthropogenic (Human-Made) Sources

Human activities contribute significantly to NMHC emissions through the combustion of fossil fuels, vehicular exhaust, industrial emissions, oil refineries, petrochemical industries, biomass burning, solvent use, paint industries, waste burning, and fuel evaporation from petrol stations.

Formation and Atmospheric Behaviour

NMHCs react rapidly with hydroxyl (OH) radicals, nitrogen oxides (NOx), and sunlight, initiating a series of photochemical reactions.

These reactions produce tropospheric (ground-level) ozone, secondary organic aerosols (SOAs), and other oxidants that influence air quality, visibility, and climate.

Major Atmospheric Functions

Formation of Tropospheric Ozone

NMHCs are among the most important precursors of ground-level ozone, which is a harmful air pollutant and a greenhouse gas.

Unlike stratospheric ozone, which protects Earth from ultraviolet radiation, tropospheric ozone damages crops, forests, ecosystems, and human health.

Formation of Secondary Organic Aerosols (SOAs)

NMHCs undergo oxidation in the atmosphere, producing secondary organic aerosols, which form a major component of PM₂.₅ (fine particulate matter).

These aerosols reduce visibility, influence cloud formation, and contribute to respiratory diseases.

Consumption of Hydroxyl (OH) Radicals

NMHCs react readily with hydroxyl radicals (OH), which are known as the "detergent of the atmosphere" because they remove many atmospheric pollutants.

By consuming OH radicals, high NMHC concentrations can reduce the atmosphere's ability to cleanse itself, allowing other pollutants to persist for longer periods.

Environmental and Health Impacts

• High concentrations of NMHCs contribute to photochemical smog, reduce air quality, increase ground-level ozone, and enhance the formation of fine particulate matter (PM₂.₅).

• Long-term exposure can cause respiratory diseases, eye irritation, cardiovascular problems, and increase the risk of certain cancers, especially in the case of aromatic hydrocarbons such as benzene.

• NMHCs also indirectly influence climate change by contributing to ozone formation and aerosol production.

Importance of Monitoring NMHCs

Monitoring NMHCs helps scientists understand the sources of air pollution, assess regional atmospheric chemistry, improve air-quality forecasting, and evaluate the effectiveness of pollution-control measures.

Continuous observations also support climate models, help identify pollution transport pathways, and strengthen environmental policy formulation.

ARIES Study at Munsyari

The Aryabhatta Research Institute of Observational Sciences (ARIES) conducted continuous measurements of NMHCs at Munsyari, Uttarakhand, from 2022 to 2023.

The study aimed to analyse the seasonal variation of NMHCs in the Central Himalayan region, understand their contribution to ozone formation, and evaluate their impact on regional air quality and human health.